On 05/03/2012 07:22 PM, Avi Kivity wrote:
> Currently we flush the TLB while holding mmu_lock. This
> increases the lock hold time by the IPI round-trip time, increasing
> contention, and makes dropping the lock (for latency reasons) harder.
>
> This patch changes TLB management to be usable locklessly, introducing
> the following APIs:
>
> kvm_mark_tlb_dirty() - mark the TLB as containing stale entries
> kvm_cond_flush_remote_tlbs() - flush the TLB if it was marked as
> dirty
>
> These APIs can be used without holding mmu_lock (though if the TLB
> became stale due to shadow page table modifications, typically it
> will need to be called with the lock held to prevent other threads
> from seeing the modified page tables with the TLB unmarked and unflushed)/
>
> Signed-off-by: Avi Kivity <avi@xxxxxxxxxx>
> ---
> Documentation/virtual/kvm/locking.txt | 14 ++++++++++++++
> arch/x86/kvm/paging_tmpl.h | 4 ++--
> include/linux/kvm_host.h | 22 +++++++++++++++++++++-
> virt/kvm/kvm_main.c | 29 ++++++++++++++++++++---------
> 4 files changed, 57 insertions(+), 12 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/locking.txt b/Documentation/virtual/kvm/locking.txt
> index 3b4cd3b..f6c90479 100644
> --- a/Documentation/virtual/kvm/locking.txt
> +++ b/Documentation/virtual/kvm/locking.txt
> @@ -23,3 +23,17 @@ Arch: x86
> Protects: - kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
> - tsc offset in vmcb
> Comment: 'raw' because updating the tsc offsets must not be preempted.
> +
> +3. TLB control
> +--------------
> +
> +The following APIs should be used for TLB control:
> +
> + - kvm_mark_tlb_dirty() - indicates that the TLB is out of sync wrt
> + either guest or host page tables.
> + - kvm_flush_remote_tlbs() - unconditionally flush the tlbs
> + - kvm_cond_flush_remote_tlbs() - flush the TLBs if previously marked
> +
> +These may be used without mmu_lock, though kvm_mark_tlb_dirty() needs to be
> +used while holding mmu_lock if it is called due to host page table changes
> +(contrast to guest page table changes).
In these patches, it seems that kvm_mark_tlb_dirty is always used
under the protection of mmu-lock, yes?
If both kvm_mark_tlb_dirty and kvm_cond_flush_remote_tlbs are use
out of mmu-lock, i think we can use kvm_flush_remote_tlbs instead.
If it is so, dirtied_count/flushed_count need not be atomic.
And, it seems there is a bug:
VCPU 0 VCPU 1
hold mmu-lock
zap spte which points to 'gfn'
mark_tlb_dirty
release mmu-lock
hold mmu-lock
rmap_write-protect:
see no spte pointing to gfn
tlb did not be flushed
release mmu-lock
write gfn by guest
OOPS!!!
kvm_cond_flush_remote_tlbs
We need call kvm_cond_flush_remote_tlbs in rmap_write-protect
unconditionally?
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